Actor Struan Rodger will be joining the Game of Thrones cast for the series' fourth season, according to Zap2It. His online résumé lists his character as "Three-Eyed Raven," which suggests he may be portraying a character known as Bloodraven from A Dance of Dragons, the fifth novel in author George R. R. Martin's A Song of Fire and Ice series, which serve as the basis for the HBO epic.
(Spoilers ahead.) As Zap2It explains, a raven with three eyes has already appeared in the storyline surrounding Bran Stark, so it stands to reason that Rodger would be portraying the person Meera and Jojen Reed are leading him to. In the books, Bloodraven is Lord Brynden Rivers, the last "greenseer," who lives in a cave north of the Wall and appears in Bran Stark's dreams as a three-eyed crow.
Bloodraven would be the most pivotal character in Bran's journey at the point in the story. But because the character's introduction comes so late in the novels, at least compared with the storyline the series is following, it's possible that he would be introduced in Stark's visions and possibly brought in in a larger way in the fifth season.
Game of Thrones will return to HBO in spring 2014.